I've noticed that you can't smooth a wall on a diagonal, although the diagonal still contributes to the room's overall beauty. Any chance that this can be fixed? If I recall correctly, you used to not be able to build a wall on the diagonal, but you can now. I'd assume it's the same kind of mechanic.
Otherwise it's really fiddly to have to mine out another section of wall, smooth the corner, and then build a wall in the spot you mined out.
Otherwise it's really fiddly to have to mine out another section of wall, smooth the corner, and then build a wall in the spot you mined out.